Hip prosthesis provided with a shaft inserted into the femur

ABSTRACT

A hip prosthesis includes a shaft which can be inserted into a hole in the femur without using cement. The proximal section of the shaft includes a base body having a form that is complementary to the shape of the hole in the femur into which the shaft is to be inserted. The hole is made using a tool of a shape corresponding to the base body. The base body has a cuneus rib raised on its dorsal or ventral surface that has a width in its center that is at least three times its height. The rear surface of the cuneus rib has rough and abrasive properties.

FIELD AND BACKGROUND OF THE INVENTION

The part of a hip prosthesis assigned to the femur is equipped with ashaft inserted into a cavity which, after resection of the head and neckof the hip, is formed by a suitable tool in the spongy innercross-sectional area of the femur. If the shaft is inserted withoutusing cement, it is endeavored to make the shape of the cavity as far aspossible complementary to the shape of the shaft, so that the prosthesishas a secure and firm fit after insertion of the shaft. The proximalportion of the shaft coming to lie in the metaphyseal region of the bone(approximately above the lesser trochanter) is designed in such a waythat it can transmit not only vertical forces in the direction of thefemur but also forces extending transverse thereto and, in particular,medially directed forces. To ensure that the surface areas of theproximal prosthesis shaft oriented in the ventral and dorsal directionsalso take part in transmission of forces to the bone, it is known todesign their surface such that a form-fit connection with the bonetissue can be obtained. Two possibilities are available for this,namely, on the one hand, a surface micro-roughness which is broughtabout, for example, by glass blasting, porous coating or the like, andallows the bone to infiltrate into the depressions and pores, and, onthe other hand, ribs which project from the base body of the shaft.These two possibilities can also be used together. Thus, it is known(EP-B-761 183; U.S. Pat. No. 5,755,811) to arrange ribs on the dorsaland ventral faces of the proximal base body of the shaft, said ribsextending in the longitudinal direction of the shaft, and their crosssection increasing in a wedge shape from distal to proximal. Such ribsare referred to below as wedge-shaped ribs. The cavity is produced usinga rasp whose shape corresponds to the base body of the shaft without theribs. Upon insertion of the shaft, the base body of the shaft forms apress fit with the surface of the cavity. The wedge-shaped ribs cut intothe spongy bone tissue as the prosthesis is being inserted. Because oftheir wedge shape, they displace and compact the bone tissue. Thiscontributes to the secure fit of the prosthesis. First, a macroscopicform fit is thus obtained between the bone and the prosthesis crosssection, by virtue of the rib. Second, a microscopic form fit isobtained after bone tissue has grown into the rough or porous surfacestructure. These two effects evidently occur independently of oneanother and are used independently of one another in the construction ofa prosthesis.

Of course, the said compression of the bone material during insertion ofa wedge-shaped rib must not lead to the bone breaking. This danger isall the greater, the wider a rib, because the latter then directs agreater wedge surface toward the cortical bone and produces a greaterwedging force against the cortical bone. Known wedge-shaped ribs aretherefore made of a narrow design (EP-B-761 183, EP-B-159 462). For thesame reason, it is sought to deflect the wedging action of the ribs intothe tangential direction (DE-U-295 22 382, page 4, line 22). It is truethat wide ribs are also known in hip prostheses intended for cementlessimplantation (EP-A-10 70 490; EP-A-567349). However, these are ribswhich, because of their shape, are unsuitable for displacement andcompression of bone material and therefore require a shaping of thecavity such that their volume and their shape are taken intoconsideration from the outset in the cavity. Relatively wide shaftprojections broadening from distal to proximal are also known forprosthesis shafts which are intended to be implanted using cement and inwhich, therefore, the associated tool is designed such that theartificial bone cavity is more voluminous than the shaft inclusive ofits rib-like attachments.

SUMMARY OF THE INVENTION

The object of the invention is to make available a prosthesis shaft forcementless implantation with wedge-shaped ribs making up a rib assemblyand a rough surface, said prosthesis shaft permitting rapid andextensive adherence between the prosthesis surface and the bone tissue.The solution according to the invention lies in the features of asdisclosed herein, that the wedge-shaped rib assembly is on average atleast three times as wide as it is high and the roughness on its rearface is sharp-edged with a peak-to-valley height of between 0.05 and 0.5mm.

Based on previous experience, one has to expect that a wide wedge-shapedrib of this kind will deploy its wedging effect mainly outwardly towardthe cortical bone and that, therefore, there will be a danger of thebone breaking. This danger would in fact arise if the rear face of therib were not rough. The roughness means that the bone substance whichcomes into direct contact with the rear face of the rib, and which isexposed to the wedging effect and relative movement, is abraded andcrushed and thus brought into a flow-able state so that it can bedisplaced to the sides away from the rear face of the rib and can flowoff. This has two consequences. First, the force generated in thedirection perpendicular to the rear face of the rib is comparativelysmall, as a result of which the possibly damaging effect of the force onthe cortical bone is limited. Second, the rear face of the rib, in thefinal state of implantation, is situated in immediate proximity toundamaged bone substance in which the natural vascular system ispreserved, since the previously crushed bone substance has been removedby virtue of its flowability. This means that the process ofinfiltration of new bone substance reaches the porous rear face of therib very quickly after implantation and that, as a result, an intimateand extensive surface union is created after just a short time. It maywell be that such an effect also occurred in the previously knownwedge-shaped ribs with rough surface at their cross-sectional tip.However, since this effect was limited to a very small surface area, itwas not appreciable and it also made no positive contribution. At therib flanks, the situation is fundamentally different. In the case ofindividual ribs, a comparatively wide compression space is present inthe compression direction perpendicular to the flank surface, so thatalthough the bone substance is compressed and partially squeezed, thepressing is not as great as would be required for an appreciableabrasive effect of the roughness. The bone substance remains more orless in situ. Since the vessels therein are for the most part destroyed,it initially forms a barrier between the undamaged bone tissue and theprosthesis surface through which, after implantation, fresh bone tissuemust first have penetrated before it can infiltrate the rough surface ofthe prosthesis and bring about a form-fit connection. In the intersticesbetween adjacent ribs, the situation is once again different than at therear of the rib, because there the compression and tissue destruction isparticularly pronounced, without the destroyed tissue being able to beremoved. In this area too, the connection to fresh bone tissue cantherefore take place only with some delay.

The invention thus affords the advantage that, by virtue of theroughness of the large rear face of the rib, and because of theresulting closeness of undamaged bone tissue, a firm connection betweenprosthesis surface and bone surface can take place very quickly.

In the whole of the proximal portion, the rib is preferably at leastthree times as wide, preferably four times as wide, as it is high. Toensure that the abrasive action of the roughness during the relativemovement of the rib surface with respect to the pressed-on bone tissueis at all times sufficient compared to the squeezing generated by thewedging effect, the wedge angle, that is to say the angle between thesurface of the rear of the rib and the midplane extending in thelateral-medial direction (LM midplane) of the shaft, must not be greaterthan 5°, preferably not greater than 3.5°, and more preferably notgreater than 2.5°.

The width of the rib too, that is to say its dimension in the LMdirection, preferably increases from distal to proximal, and the anglebetween the lateral edge and the longitudinal direction of the shaftmust not be greater than 4°, preferably 3°. The same applies to thelateral edge.

In the shaft cross section, the rear face of the rib must extendapproximately parallel to the LM midplane. The angle between the rearface of the rib and the LM midplane is preferably not greater than 15°,the rib height increasing toward the lateral face. The medial flankexpediently extends from the rear face of the rib to form a sharp edgeand runs substantially perpendicular to the LM plane. The sameexpediently applies to the lateral edge too, although it is lessimportant there.

To ensure that the abrasive effect of the roughness is sufficient in thearea of the rear of the rib, the distance between adjacent peaks of theroughness is expediently of the same order of magnitude as thepeak-to-valley height, namely between 0.05 and 0.5 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below with reference to thedrawing which depicts an advantageous illustrative embodiment and inwhich:

FIG. 1 shows a side view,

FIG. 2 shows a view from the lateral direction, and

FIGS. 3-5 show cross sections through the prosthesis shaft atrespectively corresponding heights thereof.

DETAILED DESCRIPTION OF THE INVENTION

The prosthesis is a straight shaft prosthesis, that is to say the shafthas a continuously straight longitudinal axis and is implanted in arectilinear direction into the femur. The shaft 1 comprises an in crosssection substantially rectangular base body with parallel dorsal andventral faces 3, 4 which taper in a wedge shape in the distal direction.They each enclose an angle of less than 2° with the lateral-medialmidplane 5. The lateral and medial boundary faces 6, 7 of the base bodyof the shaft likewise taper in a wedge shape in the distal direction.The proximal end is adjoined by the prosthesis neck, having a cone 9 forattachment of a ball joint, and by a lateral wing 10 in the region ofthe greater trochanter.

A rasp (not shown) for forming the bone cavity intended to receive theprosthesis shaft has the same shape as the base body of the prosthesisshaft delimited by the faces 3, 4 and 6, 7, as is generally known, inorder to give the prosthesis shaft a secure and firm fit in the bonecavity after implantation.

On the dorsal and ventral faces 3, 4 of the base body of the shaft, andapproximately centrally with respect to the shaft axis 11, ribs 12 areattached which have the shape of a wedge with straight boundary faces.The size of these faces is the dorsally or ventrally oriented rear face13. Medially, the rib is delimited by a medial end face extendingapproximately perpendicular to the surface 3 or 4. The same applies tothe lateral end face 15.

With the center line 11, the end faces 14, 15 enclose, like the boundaryedges of the rear face 13, an angle of in each case approximately 2.5°.The angle which the rear faces 13 of the rib enclose with the LMmid-plane of the shaft is 2°.

The rear face 13 of the rib extends approximately parallel to the LMmidplane 5 of the shaft. In the example shown, the deviation amounts toless than 10°. At the medial edge, the rib is slightly higher than atthe lateral edge, thereby increasing the macroscopic form-fit fortransmitting force from the prosthesis medially to the bone.

At least the rear face 13 of the rib 12 is provided with a rough and, ifappropriate, porous surface by means of sandblasting, plasma coating,flame spraying or the like. The roughness elevations are sharp-edged sothat, when the shaft is pushed into the bone, they act abrasively on thebone substance. Such a roughness can also be provided on the other facesof the prosthesis shaft in order to permit intimate connection of thebone tissue with the prosthesis surface. As far as the invention isconcerned, what matters is simply the roughness of the rear face 13 ofthe rib. Whereas the base body of the shaft has been provided with acomplementary shape in the bone cavity and acquires the desired pressfit therein without any appreciable additional deformation of the bone,there is no such complementary cavity form for the ribs 12. When theshaft is introduced into the bone, the ribs displace an amount of thebone tissue corresponding to their volume. If the rear face were smooth,the laminar structure of the bone would simply be compressed andcompacted, in which case the liquid content of the lamellar intersticeswould escape. Thanks to the abrasive quality of the back of the rib, thebone lamellas in the case of the invention are abraded and cut. In thisway, they can escape with the liquid interstitial content from the areabetween the rear face of the rib and the solid and undamaged bonetissues lying behind them. On the one hand, this reduces the pressingthat arises because of the wedge effect of the ribs between these andthe bone. On the other hand, the spongy bone substance located betweenthe rear face of the rib and the hard cortical bone is not totallycompressed and damaged. Instead, it remains intact, almost as far as therib surface, and can therefore contribute to rapid securing of theprosthesis by means of fresh bone tissue rapidly advancing to the shaftsurface and infiltrating the surface roughness thereof.

1. A hip prosthesis set comprising: a hip prosthesis having a shaftwhich is configured to be inserted into a femoral cavity without usingcement, a proximal portion of the shaft comprising a base body, a firstrib projecting dorsally from the base body relative to an implantedposition and a second rib projecting ventrally from the base bodyrelative to the implanted position, each of the ribs having two flanksand a rear face and extending parallel to the shaft direction,increasing in height from a distal portion of the shaft to the proximalportion for displacing and compressing spongy bone tissue as theprosthesis is inserted into the femoral cavity and having a roughsurface to encourage bone ingrowth, and a tool which is configured toshape the cavity so that it substantially corresponds to the shape ofthe base body, wherein each of the ribs is on average at least threetimes as wide as it is high and the rough surface has a roughness on itsrear face that is sharp-edged with a peak-to-valley height range ofbetween 0.05 and 0.5 mm.
 2. The hip prosthesis set as claimed in claim1, wherein the hip prosthesis is a straight shaft prosthesis.
 3. The hipprosthesis set as claimed in claim 1 or 2, wherein the rear face of eachof the ribs extends, in a shaft cross section, approximately parallel toa lateral-medial midplane of the shaft relative to the implantedposition.
 4. The hip prosthesis set as claimed in claim 3, wherein theheight of each of the ribs, at an edge delimiting it medially, isgreater than at its lateral boundary.
 5. The hip prosthesis set asclaimed in claim 4, wherein a medial end face of each of the ribsextends approximately perpendicular to the surface of the base body ofthe shaft.
 6. The hip prosthesis set as claimed in claim 3, wherein amedial end face of each of the ribs extends approximately perpendicularto the surface of the base body of the shaft.
 7. The hip prosthesis setas claimed in claim 1 or 2, wherein a medial end face of each of theribs extends approximately perpendicular to the surface of the base bodyof the shaft.
 8. The hip prosthesis set as claimed in claim 1, whereinthe distance between the peaks of the elevations forming the roughnesson the rear face of each of the ribs is of the same order of magnitudeas the peak-to-valley height.
 9. The hip prosthesis set as claimed inclaim 1, wherein only a single first rib and a single second rib projectfrom the base body.